1. Field of the Invention
This invention relates to a power supply circuit for a motor in which a power source to a motor load can be opened and closed by a switch or the like.
2. Related Background Art
Generally, a motor used in a laser beam printer requires great power because it drives a driving system for a photosensitive drum and a paper conveying system.
A power supply circuit for a motor for driving such a laser beam printer has heretofore been such as is shown in FIG. 7 of the accompanying drawings.
In FIG. 7, the reference numeral 1 designates switch for closing and opening a power source, the reference numeral 2 denotes motor, a reference numeral 3 designates a motor drive circuit, and reference numeral 4 denotes a capacitor.
The switch 1 has, for example, the function of protecting the user, and is designed to be opened in response to the opening of the cover or the like of an apparatus such as a printer in which the motor 2 is used, and stops the power supply to the motor so that the user may not be injured even if he touches a gear or the like driven by the motor 2.
With such a power supply circuit, it is sometimes the case that when the motor 2 is driven with the switch 1 closed, a noise-like great current such a spike current flows, and because the power of a motor for LBP is large, a large ripple current flows and thus, in order to stabilize the source voltage, the capacitor 4 of large capacitance (e.g., several hundred .mu.F) is provided near the drive circuit 3.
FIG. 8 of the accompanying drawings is a timing chart showing the fluctuating states of the supplied voltage V.sub.1, capacitor current i.sub.c and source voltage (the reference value=+24V) when the switch 1 is opened and closed in the power supply circuit of FIG. 7.
Now, in the conventional power supply circuit as previously described, the capacitor 4 is provided immediately behind the switch 1 and therefore, there have been the following technical problems to be solved.
i) As shown by the capacitor current in the middle graph in FIG. 8, an excessive current flows during the closing of the switch and therefore, a spark sometimes flashes at the contacts of the switch 1 to cause burning, bad contact, or the like, of the contacts.
ii) By such excessive current during the closing of the switch, the source voltage sometimes momentarily varies during the closing of the switch as shown by the source voltage as shown in the bottom graph in FIG. 8, thereby adversely affecting other circuits.
Also, the overcurrent protecting function of the power source has sometimes been performed to shut down the power source.
On the other hand, a device in which to suppress such a current when an electronic unit is mounted on a power supply device comprises a switch circuit is provided between a power source and an electronic circuit in the electronic unit, and a current limiting circuit provided parallel to the switch circuit. During the mounting of the electronic unit, the power supply to the electronic circuit is effected through the current limiting circuit, and when it is detected by a voltage detecting circuit that the potential of the electronic circuit has reached a predetermined level, the power supply to the electronic circuit is effected through the switch circuit. Such a device is disclosed in Japanese Utility Model application Laid Open No. 59-191844.
However, it has not been known to use such a circuit for suppressing the rush current during the closing of the power source in a motor for driving a laser beam printer.